1. Field of the Invention
The invention relates to a filtration device, especially one for maintaining the quality of water in a closed loop or semi-closed loop aquaculture system.
2. Description of the Prior Art
There has been increasing interest of late in the cultivation of fish, both by the commercial farmer interested in providing fish for food, and by the hobbyist raising such fish as Koi for their beauty. While such aquaculture can take place in naturally occurring bodies of water, this is geographically limiting and requires a source of water free of contaminants which is of the proper temperature and composition for the particular fish being cultivated. Moreover, this method is generally unacceptable for adequate viewing of fish which are raised for their aesthetic value.
In the cultivation of fish and seafood, it is far more efficient to use a closed loop or semi-closed loop system in which the fish are cultivated in a tank with the water recirculated and purified. This enables fish to be cultivated long distances from suitable bodies of water and limits the amount of fresh water must be added to the system.
The use of a closed or semi-closed system requires constant filtration and treatment of the water to enable those conditions which will allow the fish to survive and flourish. Filtration of the water associated with the closed system involves both a physical filtration to remove solid matter from the water as well as treatment which will remove dissolved chemical contaminants from the water. The waste products of fish ultimately generate ammonia which is highly toxic to fish and other aquatic animals. In a closed or semi-closed system, it is therefore necessary to provide a means for removing ammonia and other nitrogenous wastes from the water. Generally, this involves culturing aerobic bacteria which utilize nitrogenous wastes in their nutritional cycle, and convert these wastes to substantially less toxic compounds. While creation of these bacterial colonies has been efficiently accomplished in the small "fish tank" type filtration systems, problems exist in attempting to apply these principles to larger systems which may contain thousands of gallons of water and are exposed to the elements.
For healthy growth of aquatic aerobic bacterial colonies, a number of conditions must be met. An adequate oxygenation of the water is crucial, since these organisms are oxygen dependent. Closely related to this is adequate flow rates of water since stagnation of water through a filtration medium will result in oxygen depletion and will encourage the growth of anaerobic bacteria which are hazardous to the health of the fish. Channeling of the water in the filter results in the exclusion of aerated water from certain regions within the filter with the formation of dead spaces which encourage anaerobic bacterial growth.
Under appropriate conditions, aerobic bacteria will colonize a variety of media. Generally, the greater the surface area exposed to adequate flow rates and thus adequate levels of oxygen, the greater the number of colonies in a given volume of media. A given volume of sand, for example, will theoretically accommodate a larger number of bacterial colonies than the same volume of stones or crushed rock, since the sand provides a larger surface area. The problem in applying this principle is that the smaller the diameter of the particulate media, the more difficult it is to provide an even flow, prevent channeling and keep the media functionally clean.
The regions of the media which first receive the inflowing water with highest oxygen saturation are the areas with the maximum number of colonies. In fact, the greatest portion of the bacterial activity in a downflow filtration device is actually at the surface of the filter in close association with detritus which is often flushed away when conventional filters are backflushed.
When designs have been proposed in the art for biological filters to be used in aquaculture. For example, U.S. Pat. Nos. 3,973,519 and 3,957,017 utilize a filter in which water flows upwardly into a cylinder containing a porous medium through a perforated plate. Water flowing to the filters is aerated at the base of the filter.
U.S. Pat. No. 3,661,262 describes a biological filter in which the water flows downwardly through a bed of sand and shells supported by a water-permeable substrate mesh or framework which permits the water to drain therethrough by gravity.
U.S. Pat. No. 3,662,889 discloses a filter which is actually immersed in a pond of water and which includes perforated concentric cylinders containing a filtration medium therebetween. The water flows inwardly through the filtration medium to the inner cylinder from which it is pumped upwardly and outwardly.
In the use of the above biological filters, one of the problems which exists is in cleaning the filter. U.S. Pat. No. 3,662,889 specifically discloses cleaning the filter by disassembling the device and detaching the filter material. In many cases, the filter can be cleaned by simply backflushing. However, in both cases, the cleaning process results in the loss of many of the bacterial colonies and the necessity for reestablishing these bacterial colonies. It has been proposed, in fact, to utilize two entirely separate, independently functional filtration systems which can be backflushed at different times, to minimize bacterial colony loss.